A developable latent image is formed in a silver halide emulsion layer of a radiographic element when it is imagewise exposed to X-radiation. Silver halide emulsions, however, more efficiently absorb and consequently are more responsive to longer (300 to 1500 nm) wavelength electromagnetic radiation than to X-radiation. Silver halide possesses native sensitivity to both the near ultraviolet and blue regions of the spectrum and can be sensitized readily to the green, red, and infrared portions of the electromagnetic spectrum.
Consequently it is an accepted practice to employ intensifying screens in combination with silver halide radiographic elements. An intensifying screen contains on a support a phosphor layer that absorbs the X-radiation more efficiently than silver halide and emits to the adjacent silver halide emulsion layer of the radiographic element longer wavelength electromagnetic radiation in an image pattern corresponding to that of the X-radiation received.
The most common arrangement for X-radiation exposure is to employ a dual coated radiographic element (an element with silver halide emulsion layers on opposite sides of a support), each emulsion layer being mounted adjacent a separate intensifying screen. The radiographic element is a consumable, used to record a single imagewise exposure, while the intensifying screens are used repeatedly.
If the luminescence of an intensifying screen persists after imagewise exposure to X-radiation has been terminated, there is a risk that the afterglow will expose the next radiographic element brought into contact with the screen. Thus, the measure of a satisfactory intensifying screen is not only the intensity of the luminescence it exhibits upon exposure to X-radiation, but also the rapidity with which the luminescence decays upon the termination of X-radiation exposure.
Of the many different phosphor compositions known, most have failed to satisfy the practical demands of intensifying screen application for failing to generate sufficient emission intensity upon exposure to X-radiation, for exhibiting persistent luminescence after exposure (afterglow), or a combination of both.
Phosphors employed in intensifying screens consist of a host compound, often combined with a small amount of another element that changes the hue and/or improves the efficiency of fluorescence. It has been recognized that useful phosphors are those in which the host compound contains at least one higher atomic number element to facilitate absorption of the high energy X-radiation. For example, barium sulfate, lanthanide oxyhalides and oxysulfides, yttrium tantalate, and calcium tungstate, are widely employed phosphor host compounds.
From time to time various compounds of zirconium and hafnium have been investigated as phosphors. Zirconium and hafnium are known to be atoms of essentially similar radii, 1.454 .ANG. and 1.442 .ANG., respectively. Practically all known compounds of zirconium and hafnium correspond to the +4 oxidation state. The chemical properties of the two elements are essentially identical.
Hale U.S. Pat. No. 2,314,699, issued Mar. 23, 1943, discloses a method of preparing a luminescent material which comprises dispersing an oxide of an element chosen from the group consisting of beryllium, magnesium, zinc, and zirconium in a solution of a salt of an element chosen from the group consisting of silicon, germanium, titanium, zirconium, hafnium, and thorium, and precipitating the dioxide of the element of the second named group upon the oxide of the element of first named group.
Leverenz U.S. Pat. No. 2,402,760, issued June 25, 1946, discloses a crystalline luminescent material represented by the general formula: EQU u(BeO)v(XO.sub.2)w(YO.sub.2):xMn
where X is a metal selected from the group of metals consisting of zirconium, titanium, hafnium, and thorium, Y is an element selected from the group of elements consisting of silicon and germanium, the molar ratio EQU u/v
being from 1/99 to 99, the molar ratio of EQU u+v/w
being from 1/3 to 2, and the sum of u+v being equal to one gram molecular weight.
Zirconium and hafnium containing compounds also containing rare earth elements have also been disclosed from time to time:
Anderson U.S. Pat. No. 3,640,887, issued Feb. 8, 1972, discloses transparent polycrystalline ceramic bodies composed of oxides of thorium, zirconium, hafnium, and mixtures thereof with oxides of the rare earth elements 58 through 71 of the Periodic Table optionally additionally including yttria. Anderson contains no mention of luminescence.
Mathers U.S. Pat. No. 3,905,912, issued Sept. 16, 1975, discloses a hafnium phosphate host phosphor with an activator selected from among terbium, praseodymium, dysprosium, thulium, and europium.
Kelsey, Jr. U.S. Pat. No. 4,006,097, issued Feb. 1, 1977, discloses ytterbium activated hafnia phosphors.
Chenot et al U.S. Pat. No. 4,068,128, issued Jan. 10, 1978, discloses as a phosphor for luminescent intensifying screens (Hf.sub.1-x Zr.sub.x)O.sub.2 :P.sub.2 O.sub.5, where x is in the range of from 0 to 0.5. Eu.sup.+2 is disclosed to enhance blue emission.
Chenot et al U.S. Pat. No. 4,112,194, issued Sept. 5, 1978, discloses as a phosphor for luminescent intensifying screens (Hf.sub.1-x Zr.sub.x).sub.3-y A.sub.4y (PO.sub.4).sub.4, where x is within the range of about 0.005 to 0.5, A is selected from the group consisting of lithium, sodium, and potassium, and y is within the range of 0.4 to 2.0. Eu.sup.+2 is disclosed as an activator for a green emitting phosphor.
Alexandrov et al U.S. Pat. No. 4,153,469, issued May 8, 1979, discloses as artificial precious stones or laser elements monocrystals of zirconium or hafnium oxide stabilized with yttrium oxide.
Klein et al U.S. Pat. No. 4,295,989, issued Oct. 20, 1981, discloses a cubic yttria stabilized hafnia phosphor doped with Ce.sup.3+.
E. Iwase and S. Nishiyama, "Luminescence Spectra of Trivalent Rare Earth Ions", Proc. Intern. Sym. Mol. Struct. Spectry., Tokyo, 1962, A-407-1 to 7, report the crystal lattice constants of monoclinic hafnia and zirconia as follows:
TABLE I ______________________________________ Oxide a-axis b-axis c-axis .beta. ______________________________________ HfO.sub.2 5.11 5.14 5.28 99.degree. 44' ZrO.sub.2 5.21 5.26 5.375 99.degree. 55' ______________________________________
Iwase and Nishiyama investigated hafnia and zirconia for cathodoluminescence--i.e., fluorescence response to electron bombardment. The emission characteristics of these oxides doped with trivalent samarium, praseodymium, dysprosium, terbium, and europium ions are reported.
It has been recognized that the inclusion of titanium as an activator can significantly increase the luminescence of zirconia and hafnia:
Kroger U.S. Pat. No. 2,542,336, issued Feb. 20, 1951, discloses a phosphor containing titanium as an activator and having a matrix composed of one or more of the oxides of zirconium, hafnium, thorium, germanium or tin, to which may be added either acid oxides or basic oxides or both.
L. H. Brixner, "Structural and Luminescent Properties of the Ln.sub.2 Hf.sub.2 O.sub.7 -type Rare Earth Hafnates", Mat. Res. Bull., Vol. 19, pp. 143-149, 1984, describes investigations of title phosphor host compounds. Ln is defined to include not only lanthanides, but also scandium and yttrium. After reporting the properties of Ti.sup.+4 as an activator for rare earth hafnates, Brixner states:
We also looked at this same activator in pure HfO.sub.2. Under 30 kVp Mo radiation x-ray excitation, this composition also emits in a broad band centered around 477 nm as seen in FIG. 5. This emission has an intensity of about 1.6 times that of PAR CaWO.sub.4 and could therefore be of interest as an x-ray intensifying screen phosphor, especially in light of the superior absorption of HfO relative to CaWO as seen in FIG. 6. Unfortunately, the price of optical grade HfO is so prohibitive that it cannot be used in screen applications. (Emphasis added.) PA0 At room temperature the phosphor exhibits a very rapid initial exponential decay . . . similar to CaWO.sub.4 and MgWO.sub.4 and some sulfide phosphors . . . Beyond about 20 .mu.sec, the decay rate becomes much slower and the phosphorescence is visually detectable for a few minutes. It was found that the addition of certain mineralizers or fluxes, in particular 1 mole % LiF, besides leading to an expected increase in particle size during firing, also causes an increase in the intensity of the phosphorescence although the intensity of the fluorescence is virtually the same . . .
J. F. Sarver, "Preparation and Luminescent Properties of Ti-Activated Zirconia", Journal of the Electrochemical Society, Vol. 113, No. 2, February 1966, pp. 124-128, discloses investigations of Ti.sup.+4 activation of zirconia. Sarver states: